MATERIAL SPECIFIC HEAT (J/g • °C)
Ammonia, NH3 4.70
Liquid water, H2O 4.184
Ethylene glycol, C2H602 (antifreeze) 2.42
Ice, H20 2.01
Water vapor, H2OS 2.0
Aluminum, Al 0.90
Iron, Fe 0.451
Silver, Ag 0.24
Gold, Au 0.13
signifies the resistance of a substance to a change in temperature. Each substance
has its own characteristic specific heat, which may be used to assist in its identi-
fication. Some typical values are given in Table 8.1.
Guess why water has such a high specific heat. Once again, the answer is
hydrogen bonds. When heat is applied to water, much of the heat is consumed
in breaking hydrogen bonds. Broken hydrogen bonds are a form of potential
energy (just as two magnets pulled apart are a form of potential energy). Much
of the heat added to water, therefore, is stored as this potential energy.
Consequently, less heat is available to increase the kinetic energy of the water
molecules. Since temperature is a measure of kinetic energy, we find that as
water is heated, its temperature rises slowly. By the same token, when water is
cooled, its temperature drops slowly—as the kinetic energy decreases, mole-
cules slow down and more hydrogen bonds are able to re-form. This in turn
releases heat that helps to maintain the temperature.
C O N C E P T C H E C K
Hydrogen bonds are not broken as heat is applied to ice (providing the
ice doesn't melt) or water vapor. Would you therefore expect ice and
water vapor to have specific heats that are greater or less than that of
C H E C K Y O U R A N S W E R As Table 8.1 shows, the specific heats of ice and
vapor are about half that of liquid water. Only liquid water has a remarkable specific
heat. This is because the liquid phase is the only phase in which hydrogen bonds are
continually breaking and re-forming.
Global Climates Are Influenced by Water’s High
The tendency of liquid water to resist changes in temperature improves the
climate in many places. For example, notice the high latitude of Europe in
Figure 8.35. If water did not have a high specific heat, the countries of Europe
would be as cold as the northeastern regions of Canada, because both Europe and
Canada get about the same amount of sunlight per square kilometer of surface
area. An ocean current carries warm water northeast from the Caribbean. The
water holds much of its thermal energy long enough to reach the North Atlantic
off the coast of Europe, where the water then cools. The energy released,
4.184 joules per degree Celsius for each gram of water that cools, is carried by the
westerly winds (winds that blow west to east) over the European continent.
The winds in the latitudes of North America are westerly. On the western
coast of the continent, therefore, air moves from the Pacific Ocean to the land.